Resonant cavity having adjacent coupling elements to provide a rejection frequency



7, 1970 R. s. KOMMRUSCH 3'5 RESONANT CAVITY HAVING ADJACENT COUPLINGELEMENTS To PROVIDE A REJECTIQN FREQUENCY' Filed Sept. 15, 1967 Z'SheetSY-Sfi'e et 1 PRIOR ART PRIOR ART TRANSMITTER RECEIVER TANsQ I REC.

FIG. 4 FIG. 9

lnvenfdr Richard S. Kom'mrusch MW. m rm ATTYS.

Oct. 27, 1970 R. SIKOMMRUSCH 4 RESONANT CAVITY HAVING ADJACENT COUPLINGELEMENTS TO PROVIDE A REJECTION FREQUENCY I Filed. Sept. 15, 1967 2Sheets-Sibtifi I I I l I I I I I FIG. 8' FIG. I0

Inventor Richard S. Kornmrusc'h ATI'YS.

United States Patent O US. Cl. 333-73 8 Claims ABSTRACT OF THEDISCLOSURE The coupling elements of a resonant cavity are positionedadjacent to each other to provide a rejection frequency in addition tothe resonant frequency of the cavity. The spacing of the couplingelements determines the location of the rejection frequency with respectto the resonant frequency of the cavity.

BACKGROUND OF THE INVENTION In communications systems it is oftendesirable to provide filtering for the receivers and transmitters sothat intermodulation and splatter can be reduced, and also to permit theuse of a single antenna for a transmitter and receiver in multiplexsystems. Cavity resonators have been effectively used in such systems asfilters since they are very high Q circuits which can be easily insertedin a line connecting a transmitter or receiver with the antenna.

However, the cavity resonators heretofore used did not providedsufficient attenuation of frequencies near the resonant frequency of thecavity. Thus, in order to effectively couple a transmitter and areceiver operating at different, but closely related frequencies, to thesame antenna, a pair of cavity resonators have been required for thetransmitter and another pair for the receiver. The use of two or morecavity resonators in series in this manner increases the insertion lossof the system due to the cavities. Thus, where one cavity would normallyhave an insertion loss of 0.5 db, a pair of cavities in series wouldhave an insertion loss of 1 db.

SUMMARY It is, therefore, an object of this invention to provide animproved cavity resonator for use in a multiplex system.

Another object of this invention is to provide a cavity resonator forcoupling a receiver and a transmitter to the same antenna and having arejection frequency in addition to a resonant frequency.

Another object of this invention is to provide a cavity resonator inwhich the rejection frequency is separately variable without changingthe resonant frequency of the cavity.

In practicing this invention a cavity resonator is provided having apair of coupling elements within the cavity. The coupling elements maybe probes or loops and are closely spaced so that their interactiondevelops a rejection frequency for the cavity different from theresonant frequency of the cavity. The cavity may be tuned in the normalmanner and the rejection frequency may be varied by changing the spacingbetween the coupling elements. When the coupling elements used areprobes the rejection frequency is lower than the resonant frequency ofthe cavity and when the coupling elements are loops the rejectionfrequency is higher than the resonant frequency of the cavity resonator.

The invention is illustrated in the drawings of which:

FIG. 1 is a cross-sectional view of a resonant cavity having adjacentcoupling elements;

3,537,641 Patented Oct. 27, 1970 FIG. 2 is a plan view of a prior artcavity;

FIG. 3 is a curve showing the frequency response of the cavity of FIG.2;

FIG. 4 is a block diagram showing the use of cavities similar to thecavity of FIG. 2;

FIG. 5 are curves showing the frequency response of the cavities of FIG.4;

FIG. 6 is a plan view of the cavity of FIG. 1;

FIG. 7 is a side view of the cavity of FIG. 6 showing the use of a probeas a coupling element;

FIG. 8 is a curve showing the frequency response of the cavity of FIG.6;

FIG. 9 is a block diagram showing the use of cavities similar to thecavity of FIG. 6;

FIG. 10 are curves showing the frequency response of the cavities ofFIG. 9;

FIG. 11 is a view of the cavity of FIG. 1 showing the means foradjustably positioning the coupling element;

FIG. 12 is a cross-sectional view showing means for clamping a loop; and

FIG. 13 is a cross-sectional view showing means for clamping a probe.

DESCRIPTION OF THE INVENTION A cross-sectional view of the cavity ofthis invention is shown in FIG. 1. The cavity is illustrated asincluding an outer conductor in the form of a cylindrical can 10 havinga bottom closure 11 and a top closure 13 with a central opening 14therein. Secured in opening 14 is a tubular member 16 which extendsconcentrically into can a substantial portion of distance to bottomclosure 11. Inside tubular member 16 is a plunger 17 which combines withtubular member 16 to provide a center conductor of adjustable length.Can 10 and center conductor 16 thereby form a resonant line which isshorted at the top by closure 13. Plunger 17 is supported by anactuating rod 19 which is adjustably mounted on bracket structure 20.Bracket 20 is supported on an annular top closure 22 which is secured tocan 10 at its outer edge and having the inner diameter thereofpositioned closely about tubular member 16. A pair of nuts 23 and 25threaded on rod 19 are positioned on either side of bracket 20 to permitrod 19 and accordingly plunger 17 to be adjustably positioned withrespect to can 10. Bracket 20 includes upstanding portions 26 which aresecured to top member 22. For making the portions 26 more rigid bracingmembers 28 may be provided. A bridged member 29 connects the twoupstanding portions 26 and includes an opening 31 which receives rod 19.

As previously stated, plunger 17 is movable within the tubular member 16and includes a portion 32 which extends beyond the tubular member.Tubular member 16 includes a reduced end 34 which engages plunger 17 ina sliding fit towards the plunger and makes electrical contacttherewith. In the very high frequency range the cavity filter may beused as a quarter wave resonant line and by changing the position ofplunger 17, by adjustment of the nuts 23 and 25, the length of the linecan be changed so that the filter can be made resonant at any desiredfrequency within a predetermined range. For use in a system operating inthe frequency range from to 162 mega cycles, the overall size of thefilter is slightly more than two feet.

The resonant cavity filter includes a pair of coupling loops 36 and 37which form the input and output connections to the filter. The loops areso formed that the characteristic impedance of the loop will be equal tothat of the coaxial line to which it is connected. Coupling loop 36 isconnected to a fitting 39 and coupling loop 37 is connected to a fitting40. Fittings 39 and 40 are adapted to receive coaxial cables, thefittings being arranged so that loops 36 and 37 are connected betweenthe center 3 connector and the shield of the coaxial cable. Fittings 39and 40 also connect the shields of the cable to can 10.

In FIG. 2 there is shown a prior art cavity resonator similar inconstruction to that shown in FIG. 1. Such a cavity is described in Pat.No. 2,637,782, issued May 5, 1953, to Henry Magnuski. The cavityresonator of FIG. 2 has an outer conductor 42 and an inner conductor 43concentrically positioned with respect to outer conductor 42. A pair ofcoupling elements 45 and 46 are positioned within the cavity of thecavity resonator. In the prior art structure of FIG. 2, the couplingloops 45 and 46 are arranged opposite each other so that the anglebetween them is 180".

In FIG. 3 the frequency response curve of the cavity resonator of FIG. 2is shown. The curve of FIG. 3 shows an insertion loss of 0.5 db at theresonant frequency f and a rejection of approximately 35 db at a secondfrequency I A single cavity resonator with the rejection curve shown inFIG. 3 does not provide suflicient rejection at frequencies close to theresonant frequency so that two or more cavity resonators are coupled inseries as shown in FIG. 4. In FIG. 4 cavity resonators 48 and 49 coupletransmitter 51 to antenna 52. Antenna 52 is coupled to receiver 57 bycavity resonators 54 and 55. Conpling the cavity resonators in series asshown in FIG. 4 provides increased attenuation at frequencies close tothe resonant frequency.

Assuming the transmitter 51 is transmitting on frequency f and receiver57 is receiving on frequency fog, it can be seen from the curves of FIG.that the attenuation of resonators 54 and 55 to the transmission atfrequency f has increased to 70 db while the attenuation of resonators48 and 49 to the transmitter noise at the receiver frequency f has alsobeen increased to 70 db. By this means the transmitter signal isprevented from reaching the receiver at sufiicient amplitude to causereceiver desensitization by overloading the RF. circuits. Alsotransmitter noise signals are prevented from reaching the receiver atsuflicient amplitude to compete with the desired signals on frequency fHowever, the increase in the rejection achieved by the system of FIG. 4is gained at the expense of an increase in the insertion loss, which nowis 1 db, and also an increase in the number of cavity resonators used.This increases the cost and complexity of the system.

Referring again to FIG. 1, it can be seen that coupling loops 36 and 37are not positioned opposite each other but are adjacent. By changing theposition of the coupling loops so that they are close together, arejection frequency is developed in the cavity resonator withoutaffecting the resonant frequency. In FIG. 6 the position of the couplingloops is illustrated. The cavity resonator is similar in construction tothe cavity resonator shown in FIG. 2 and includes an outer conductor 56and an inner conductor 60. Also included are coupling loops 61 and 63.As shown in FIG. 6, lines connecting a center or axial position on theinner conductor 60 to the points at which the coupling loops 61 and 63pass through the outer conductor 56 form an angle 6 which defines theposition of the coupling loops with respect to each other. In most ofthe systems in which this cavity resonator has been incorporated, it hasbeen found that the angle 0 is equal to or less than 45. However, it hasbeen found that useful results are obtained when the angle 0 is lessthan or equal to 90.

While the structure of FIG. 1 is shown as a coupling loop, it has beenfound that coupling probes are also useful with this cavity resonator.Such a coupling probe 64 is shown in FIG. 7. While only a single probeis shown to illustrate its use, the cavity resonator of FIG. 7 wouldinclude a second probe positioned in a manner similar to the couplingloops 61 and 63 of FIG. 6. Again the angle 0 between the coupling probeis determined in the same manner as the angle 0 of FIG. 6 between thecoupling loops and is definitive of the rejection frequency. As with thecoupling loops the angles of 45 and 90 produce the same results whenapplied to the coupling probes.

The frequency response of the cavity resonator of FIG. 1 is shown inFIG. 8. Curve 66 illustrates the frequency response of a cavityresonator tuned to a frequency f and having a rejection frequency offog. The insertion loss of the cavity is the same as with the prior artcavities 0.5 db. However at the rejection frequency f the attenuation isconsiderably increased. The rejection frequency can be changed from fogto f by decreasing the angle 0, that is, by moving the two loops closertogether. This is shown in curve 67. Curve 66 represents the frequencyresponse when using coupling loops. If coupling probes were substitutedfor the coupling loops, the curve would have a shape similar to that ofFIG. 8 except that the rejection frequency would be less than theresonant frequency instead of higher than the resonant frequency.

In FIG. 9 the transmitter 69 is coupled to antenna 72 by cavity 70 andantenna 72 is coupled to receiver 75 by cavity 73. The system of FIG. 9is similar to that of FIG. 4 except that cavities 70 and 73, havingadjacent coupling elements, are used in place of the prior art cavities48, 49, 54 and 55 shown in FIG. 4.

FIG. 10 illustrates the result of using an adjacent coupling element incavity resonators 70 and 73 of FIG. 9. Curve 77 illustrates thefrequency response of cavity 70 which uses loops as coupling elements.It can be seen that, at the receiver frequency, the rejection of cavity70 is very high, at least as high or higher than the rejection of thepair of cavities 48 and 49 of FIG. 4. Curve 78 shows the frequencyresponse of cavity resonator 73 which uses probes as coupling elements.In this instance the rejection frequency is lower than the cavityresonate frequency to provide a rejection frequency at frequency f thefrequency of the transmitter 69 of FIG. 9. Only a single cavity isrequired in each of the lines connecting the transmitters and receiversto the antenna to achieve the same amount of rejection or a greateramount than was achieved by using two cavities in the prior art circuitof FIG. 4. The cavities having adjacent coupling elements introduce aninsertion loss of 0.5 db which is approximately /2 that of the pair ofthe prior art cavity resonators.

In FIG. 11 a portion of the resonant cavity is shown consisting of anouter conductor 80 and an inner conductor 81. Loops 83 and 84 arepivotally mounted on fittings 86 and 87. The other end of the loops areclamped to outer conductor 80 by clamping means 89 and 90 extendingthrough slot 92 in outer conductor 80. By this means loop 83 can bemoved to a new position 93 while loop 90 can be moved to a new position95. This movement will have the effect of increasing the angle 0 therebypermitting the rejection frequency to be varied so that the rejectionfrequency can be tuned independently of the resonant frequency of thecavity.

In FIG. 12 there is illustrated means for clamping a loop 96 to theouter conductor wall 98. A screw 99 is inserted through the slot 101 inouter conductor 98. A nut 102 together with a pair of washers 103 and104 clamp loop 96 to the outer conductor to fix it into position.

A means for clamping a probe is shown in FIG. 13. The end of probe 106is fastened to insulating material 107 by screws 109 and 110. A stud 112extends from insulating material 107 through slot 113 in outer conductor115. The stud 112 is clamped in place by means of nut 116 and Washer117.

An example of a cavity resonant at a frequency of mHz. and incorporatingthe features of the invention has the following dimensions:

Length of probes When the above cavity had its probes positionedapproximately apart the rejection frequency was 200 kHz. from theresonant frequency. When the probe separation increased to 30 thefrequency separation increased to 8 mHz.

What is claimed is:

1. A cavity resonator having a resonant frequency and a rejectionfrequency, including in combination, a cylindrical inner conductorhaving a center position thereon, an outer conductor surrounding saidinner conductor and spaced apart therefrom, the spacing and dimensionsof inner and outer conductors determining the resonant frequency, afirst coupling probe located at a first point on said outer conductorand positioned within the cavity, said proble having a portion extendingparallel to said inner and outer conductors for coupling a signal waveinto the cavity, a second coupling proble located at a second point onsaid outer conductor and positioned within the cavity, said second probehaving a portion extending parallel to said inner and outer conductorsfor coupling said signal wave from the cavity, said first and secondpoints being so located on said outer conductor that lines joining saidcenter position and said first and second points define an angle lessthan or equal to 90, the position of at least one of said first andsecond coupling probes being variable, whereby the magnitude of saidangle can be changed to thereby change the rejection frequency.

2. The gravity resonator of claim 1 wherein said angle is no greaterthan 45 3. A cavity resonator having a resonant frequency and arejection frequency, including in combination, a cylindrical innerconductor having a center position thereon, an outer conductorsurrounding said inner conductor and spaced apart therefrom, the spacingand dimensions of said inner and outer conductors determining theresonant frequency, a first coupling element located at a first point onsaid outer conductor and positioned within the cavity for coupling asignal wave into the cavity, a second coupling element located at asecond point on said outer conductor and positioned Within the cavityfor coupling said signal wave from the cavity, said first and secondpoints being so located on said outer conductor that lines joining saidcenter position and said first and second points define an angle lessthan or equal to 90, the

magnitude of said angle determing the rejection frequency, and mountingmeans for at least one of said coupling elements for varying theposition thereof with respect to the other one of said coupling elementswhereby the magnitude of said angle can be changed to change therejection frequency.

4. The cavity resonator of claim 5 wherein said angle is no greater than5. The cavity resonator of claim 5 wherein, said first and secondcoupling elements are loops extending within the cavity, whereby therejection frequency is higher than the resonant frequency.

6. The cavity resonator of claim 5 wherein, said first and secondcoupling elements are probes extending within the cavity, whereby therejection frequency is lower than the resonant frequency.

7. The cavity resonator of claim 5 wherein, said outer conductor is inthe form of a cylinder concentrically surrounding said inner conductor,the length of said inner conductor being adjustable whereby the resonantfrequency of the cavity is changed by changing said length.

8. The cavity of claim 5 wherein, each of said first and second couplingelements has a first end pivotally fixed to said outer conductor and asecond end, said outer conductor having a slot formed therein, separateadjustable fastening means mechanically connected to each of said secondends, said fastening means further being positioned in said slot andmovable therein whereby the magnitude of said angle is adjustable tovary the rejection frequency.

References Cited UNITED STATES PATENTS 2,418,961 4/1947 Wehner 3438302,637,782 5/ 1953 Magnuski.

FOREIGN PATENTS 32,749 1/1966 Germany. 337,886 6/ 1959 Switzerland.

HERMAN KARL SAALBACH, Primary Examiner W. H. PUNTER, Assistant ExaminerUS. Cl. X.R. 33382, 83

